1. Field of the Invention
The present invention relates to a process for preparing silica-filled rubber powder, silica-filled rubber powder prepared by the process, and compositions and uses including the same.
2. Discussion of the Background
A wide variety of publications have appeared concerning the preparation of rubber powders (U. Gxc3x6rl, K. H. Nordsiek, Kautsch. Gummi Kunstst. 51 (1998) 200; U. Gxc3x6rl, H. Lauer, Gummi Fasern Kunstst. 53 (2000) 261; and U. Gxc3x6rl, M. Schmitt, Paper given at ACS meeting, Rubber Division, Dallas, Tex., April 2000). Generally, these rubber poweder products are prepared via simultaneous precipitation, by means of a Lewis or Brxc3x6nsted acid, of an aqueous filler suspension (inter alia precipitated silica) and of a latex emulsion.
FIG. 1 is a diagram of the preparation and work-up of precipitated silicas as currently used to the extent of some hundreds of thousands of metric tons annually in the rubber industry (S. Wolff, Text of lecture, rubber technology studies at Hanover University).
Silica is generally produced by precipitation from an alkali metal silicate solution by means of sulfuric acid, as given in the equation below:                     (                              Na            2                    ⁢          O          xc3x97          3.3          ⁢                      xe2x80x83                    ⁢                      SiO            2                          )            +                        H          2                ⁢                  SO          4                            ⏟              Sodium        ⁢                  xe2x80x83                ⁢        waterglass              ⟶                    3.3        ⁢                  xe2x80x83                ⁢                  SiO          2                    +                        H          2                ⁢        O            +                        Na          2                ⁢                  SO          4                            ⏟      Silica      
This means that precipitation in water (precipitation suspension) gives 3.3 mol of SiO2 (silica) and 1 mol of alkali metal sulfate (generally Na2SO4). The filtration step that follows serves to remove most of the water and at the same time, in a complicated washing step, to separate the alkali metal sulfate from the silica. The silica-precipitation product from the filter press is generally termed filter cake. This includes about 20% solids and 80% water. All the water must then be removed in an energy-intensive drying process. The subsequent steps, such as milling and pelletizing, serve for further work-up of the silica so that it can be prepared as necessary and used in the rubber industry.
The publications and patents (DE 19816 972.8 and DE 198 43 301.8) which have appeared to date concerning the preparation of silicatic fillers, in particular concerning rubber powders containing silica fillers, start from salt-free washed filter cake, or even from dried silica. This means that high costs for investment in equipment and for the use of energy and other resources have been loaded onto the silica before it begins its use in the rubber powder.
Since high-activity precipitated silica fillers began to be used in tire tread mixtures, these being increasingly used since the start of the 90s, bifunctional organosilanes have been indispensable for achieving good tire properties when the aim is to improve rolling resistance (fuel saving) and wet skid resistance (EP 0,501,227; U.S. Pat. No. 5,227,425; G. Agostini, J. Bergh, Th. Materne; New Compound Technology, October 1994, Akron, Ohio, USA; and S. Wolff, The Influence of Fillers on Rolling Resistance, presented at the 129th Meeting of American Chemical Society, Rubber Division, New York, April 1986).
The most important representatives of these compounds in the rubber industry are bis(triethoxysilylpropyl)di- and -tetrasulfane (TESPD and TESPT, respectively) (U. Gxc3x6rl, Gummi Fasern Kunstst. 51 (1998) 416). The triethoxysilyl groups of both of these silanes react at elevated temperatures with the silanol groups of the silica. The user can carry out this reaction during preparation of the rubber mixture in the internal mixer or on the roll mill (U. Gxc3x6rl, A. Parkhouse, Kautschuk Gummi Kunstst. 52 (1999) 493; U. Gxc3x6rl, A. Hunsche, xe2x80x9cAdvanced Investigations into the Silica/Silane Reaction Systemxe2x80x9d, presented at the ACS meeting, Rubber Division, Louisville, Ky., USA, October 1996; and S. Wolff, xe2x80x9cThe Role of Rubber to Silica Bond in Reinforcementxe2x80x9d, presented at the First Franco-German Rubber Symposium, Nov. 14-16, 1985, Obernai, France). Another possibility is to react silica filler and organosilane with one another separately from the actual preparation of the rubber. The term then used is premodified filler (U. Gxc3x6rl, R. Panenka, Kautschuk Gummi Kunstst. 46 (1993) 538). The literature describes a wide variety of processes for this purpose.
DE 198 16 972.8 refers to a precipitation process for preparing rubber powder, in which suspensions of these fillers premodified with organosilicon compounds are prepared and stirred into the rubber emulsion.
DE 198 43 301.8 describes a process which uses a small number of stages and leads to a rubber powder with advantageous usage properties. This is based on direct addition of the silane to the silica suspension, and the silica starting material used here includes the filter cake or the dried product.
An object of the invention is to start from a salt-containing silica suspension in preparing rubber powders including precipitated silicatic fillers.
This and other objects have now been achieved by the present invention, the first embodiment of which provides a process for preparing silica-filled rubber powder, which includes contacting a salt-containing silica-precipitation suspension with at least one selected from the group including a rubber latex, rubber solution, and rubber emulsion, to form a mixture; and precipitating to produce a silica-filled rubber powder.
Another embodiment of the present invention provides a silica-filled rubber powder prepared by the above-mentioned process.
Another embodiment of the present invention provides a composition, which includes a silica-filled rubber powder, wherein the silica-filled rubber powder is prepared by the above-mentioned process.